I've been eyeing this, and the stuff thrown around... and I tried to narrow down what actually seemed to matter about this.
The Pact and Oni needs a place to be. They need a place where not much people would have run into them, while being close enough to be able to distribute goods.
I think Arieg's upper plane/negative plane/whatever is kind of beyond the point. Doesn't matter, so best just to move on from that.
At this point in time, humankind is mostly represented by Nepleslia and Yamatai. Yamatai's borders kind of fall off around the Chen nebula, becuase there are a few alien species beyond there that we havent found out. It's a location Yamatai hasn't done that much expanding into. But that border is fairly close to independent worlds like Halna, which has been a haven for independents. I'm thinking about east of the Chen nebula is actually a good spot to put it there. It's enough out of notice of Yamatai (fold is point-to-point, and the closest Yamatai might have gone to them is just traveling from Gartagen to Lorath space). Emissions from there could have been muted by the nebula, but they might not have been entirely unknown by the civilians operating in the area (and if I was independent, I wouldn't tattle-tale about a place where I can get/sell goods without the trade laws a nation likely places on merchants).
Basically, I don't think this needs this much fuss. At this point in time, the Pact hasn't received that much attention. They get placed presently, and they might be found afterwards.